1. Field of the Invention
The present invention relates to an optical film, a method for producing the same, a polarizing plate and an image display device.
2. Description of the Related Art
In various image display devices, for example, a liquid crystal display device (LCD), a plasma display panel (PDP), an organic EL display (OLED), a cathode ray tube display device (CRT), etc., an antireflection film is disposed on the surface of the display for the purpose of preventing reflection of external light or a lowering of contrast due to reflection of an image. For that reason, the antireflection film is required to have high physical strengths (for example scratch resistance, etc.), transmittance, chemical resistance, weather resistance and the like in addition to high antireflection properties.
In particular, an antireflection film with high antireflection properties is desired for the purposes of preventing the reflection of external light and enhancing the image quality in an image display device. There has hitherto been employed a method of providing an optical functional layer (optical interference layer) on a hard coat layer foamed on a plastic substrate. In order to realize higher antireflection properties, it is known to be effective to stack plural optical functional layers. However, in order to realize this by a wet coating process, it is necessary to repeat steps of coating, drying and curing several times; and thus, not only the productivity is remarkably lowered, but the coating cost increases. For example, when two layers of a low refractive index layer and a high refractive index layer are stacked as the optical functional layer, repetition of two times is necessary. In the case of additionally coating a middle refractive index layer for the purpose of further reducing the reflectance, repetition of three times is necessary. Accordingly, a technology for forming plural layers by single coating is eagerly desired.
In order to prevent the reflection of external light, a method of reducing the reflectance by providing a layer having a low refractive index as far as possible is also effective. As a method of reducing the refractive index of a layer, there is known a method of introducing a fluorine atom or introducing an inorganic fine particle having a hollow structure in the inside thereof. However, according to all of these methods, there is a tendency in which the coating strength and adhesion are impaired and the scratch resistance is lowered. Thus, it was difficult to make both low refractive index and high scratch resistance compatible with each other.
As a method for solving this problem, JP-A-2006-235198 proposes an optical film including a support having thereon an optical functional layer prepared by coating a composition containing a fine particle and a binder, wherein an SP value which is a ratio of an average particle packing factor (B) in the 30% thickness on the upper layer side on the opposite side to the support to an average particle packing factor (A) in the whole layer of the optical functional layer [(B/A)×100] is 90% or more and not more than 333%.
Also, JP-A-2007-238897 discloses a curable resin composition containing (A) a metal oxide particle having (Ab) a polymerizable unsaturated group-containing organic compound bound thereto; (B) an ethylenically unsaturated group-containing fluorine-containing polymer; (C) a particle composed of; as a major component silica having (Cb) an alkoxysilyl group-containing fluorine-containing polymer bound thereto; (D1) a solvent having high solubility against the ethylenically unsaturated group-containing fluorine-containing polymer (B); and (D2) a solvent having a lower relative evaporation rate than the solvent (D1) and having compatibility with the solvent (D1). According to the technology of JP-A-2007-238897, it is mentioned that two or more layers can be formed from a single coating film obtained by coating the subject curable resin composition.
However, the cured film obtained according to the technology of JP-A-2007-238897 involves a problem that the scratch resistance is still insufficient.